1. The Field of the Invention
The present invention is directed to systems, methods and apparatus for achieving enhanced contrast in reflective imaging systems, such as those utilizing reflective liquid crystal display imagers and color splitting devices, such as Philips prisms. The invention maximizes the transmission of light polarized in a certain direction while minimizing the transmission of light polarized in another direction, thereby achieving a high contrast ratio which significantly improves the final image quality.
2. The Relevant Technology
Liquid crystal displays are commonly used as spatial light modulators in projection imaging systems. A reflecting type of liquid crystal panel (which is also known as a liquid crystal light valve) comprises an array of pixels, which when activated works by reflecting incident light while simultaneously rotating the polarization vector of the light by 90 degrees, typically when a voltage or signal is applied to an individual pixel. Thus the signal or image information is contained in the light which is of a particular polarization. If the liquid crystal imager is not activated, then those particular pixels of the liquid crystal imager are in the xe2x80x9coffxe2x80x9d state, and the light which is reflected from them will have no rotation of the polarization state. The signals from these xe2x80x9coffxe2x80x9d pixels should correspond to dark spots in the final image. One aspect of the quality of an image in such a system is measured through a parameter known as the contrast ratio, which is defined as the ratio of the light transmitted through the system in the xe2x80x9conxe2x80x9d state divided by the amount of light transmitted in the xe2x80x9coffxe2x80x9d state. The higher the contrast ratio, the better the overall quality of the image.
Loss of contrast through a non-polarizing color splitting device such as a Philips prism results from a combination of the geometrical effect from skew rays as well as diattenuation and phase differences in the coatings of reflective and total internal reflection surfaces.
The geometrical effects of a polarizing beam splitter have been described in detail in Ootaki (U.S. Pat. No. 5,459,593) and Miyatake (U.S. Pat. No. 5,327,270), the disclosures of which are hereby incorporated by reference, as follows below. These geometrical effects are a pure rotation of the input linearly polarized light by a polarizing beam splitter. Rear projection imaging systems typically have a contrast ratios of not less than 50:1 as suggested in Ootaki, in the plots showing a 2% dark level (100%/2%=50:1).
In the Ootaki patent, white light from a halide or xenon lamp is incident at an angle of approximately 45 degrees onto a polarizing cubic beam splitter. The polarizing cubic beam splitter reflects light which is of s-polarization and transmits light which is of p-polarization (where s-polarization refers to light which has its polarization vector perpendicular to the direction of propagation, while p-polarization refers to light which has its polarization vector lying in the plane of propagation). The light which is of s-polarization is reflected by the polarizing beam splitter towards a dichroic mirror. The dichroic mirror in the Ootaki patent is designed in such a way as to reflect the s-polarized light which is of one color while transmitting the other color components of the beam. The use of more than one dichroic mirror results in a separation of the incident white light into various color channels. In a typical imaging system, two dichroic mirrors are sufficient to separate incident white light into red, green, and blue color channels. The color selectivity of the dichroic mirror is achieved by the placement of specific optical coatings upon the mirror, which is a well known technique in the art for color separation.
A limitation in the quality of the performance of this system originates from the rotation of the plane of polarization in the polarizing prism for incident light rays which are not in an eigenstate. Since this rotation is independent of the state of the image generating pixels it results in a leakage of light in the xe2x80x9coffxe2x80x9d state pixels which degrades the image contrast. The optical coatings on the dichroic mirror were designed to compensate for rotation of the polarization state.
Miyatake discloses a similar approach to compensate for the polarizing beam splitter. The approach disclosed in the Miyatake patent is to compensate for the polarizing beam splitter with a quarter waveplate in the optical path between the reflecting type liquid crystal device and the polarizing beam splitter. In Miyatake the quarter waveplate retarder was aligned with its plane perpendicular to the optical axis and is laminated to the terminal surface of polarizing beam splitters to reduce the Fresnel reflection at its interface with the prism. However this patent does not teach or consider phase differences that may occur in a color splitting device, such as a tilted dichroic mirror or a Philips prism.
In U.S. Pat. No. 5,594,591 which issued to Yamamoto et al.; the disclosure of which is hereby incorporated by reference, the inventor has attempted to solve the same problem in a projection display wherein the color separation element is a Philips prism. The Philips prism disclosed in the Yamamoto et al. patent employs optical coatings upon the faces of the Philips prism for color separation and an anti-reflection coating on the incident prism faces, which also form a total internal reflection (TIR)surfaces. Yamamoto et al. also assert that the optical coatings on the TIR surfaces, which comprise alternating layers of SiO2 and TiO2, have a phase control function. The dichroic optical coatings used for color separation cooperate with the anti-reflection coating layers at the TIR surface in achieving this phase control function; which combined the 90 degree phase difference at the TIR surface corrects for the image degradation contributed by the polarizing beam splitter.
When utilizing a quarterwave compensating plate , or quarter waveplate retarder, in the optical path between each of the three liquid crystal light valves and the Philips prism in such a reflective imaging systems, the contrast ratio is improved by ensuring that the black level is closer to being completely black. While use of quarter waveplates in such a system proposes a means for the correction of rotations in the polarization vector due to the polarizing beam splitter, it does not address the undesired ellipticity and additional rotation added by the color splitter.
A quarter wave compensation plate , or waveplate retarder is also used in U.S. Pat. No. 5,576,854 issued to Schmidt et al., the disclosure of which is hereby incorporated by reference. The Schmidt et al. patent was developed for monochromatic systems and does not address the issue of color imaging. The system disclosed in Schmidt et al. works in a manner similar to the system disclosed in Miyatake, as previously described, namely by the reduction of off-axis depolarization induced by geometric effects when the light encounters the polarizing beam splitter. Schmidt et al. specifically mentions using a waveplate with a value of retardance equal to 0.25 to compensate for the off-axis polarization components generated by the polarizing beam splitter. However, Schmidt et al. additionally suggests that an additional retardance of 0.02 be included to compensate for the unwanted polarization shifts generated by the thermally induced birefringence of the liquid crystal light valve, an effect which results in the dark state being lighter than desired. Accordingly, Schmidt et al. suggests that in monochromatic imaging systems the waveplate compensator have a total retardance value equal to 0.27 to compensate for the additional retardance, or phase delays between components due to the thermally induced birefringence in the LCLV.
In commonly assigned U.S. Pat. No. 5,986,815,incorporated herein by reference, Bryars teaches methods and apparatus for correcting undesired depolarization of color splitters through the use of uniquely designed waveplate retarders. The waveplate retarders are disposed to optimally maximize the transmission of light polarized in the xe2x80x9con statexe2x80x9d while minimizing the transmission of light polarized in the xe2x80x9coff statexe2x80x9d, for each color of the display system, thereby achieving a high contrast ratio which significantly improves the final image quality.
While the use of waveplate retarders can provide significant improvements to the contrast of projection display imaging systems, the incorporation of the retarder in the optical system introduces complications which must be overcome. Foremost among these is the Fresnel reflection that occurs as an interface between the retarder and air or between the retarder and glass surfaces to which the retarder is bonded. Although the Fresnel reflection can be significantly reduced by application of an anti-reflection coating to one or both surfaces of a plano, optical component, such as a waveplate retarder, the Fresnel can only be reduced to zero at a single or narrow band of wavelengths. The birefringence of a waveplate retarder introduces an additional complication to reducing the Fresnel reflection to zero. The anti-reflection coating can only be perfectly effective for one of the two refractive indices which corresponds to either the fast or slow axis of the birefringent material which comprises the retarder plate.
Similarly, lamination of the retarder plate to another optical component having a similar refractive index to the birefringent material will only be completely effective for light waves propagating in a direction where the effective index of the retarder perfectly matches with the refractive index of the substrate.
Additionally, waveplate retarders are commercially available in discrete thickness"", which will correspond to a quarter wave of phase retardance for light of a particular wavelength. The full benefits of the teachings of Bryars would require the manufacture of a custom waveplate retarder for each of the three color channels of a color display system. While waveplate retarders can be manufactured at any value of retardance, this results in increased complexity and logistics of manufacturing increasing cost.
It would be substantially beneficial to identify systems, methods and apparatus for further improving the contrast of projection display systems. More specifically, it would be a significant improvement in the art to minimize and correct for the rotations and ellipticity which occur in such systems that impair the contrast ratio by generating unwanted depolarization and contributing to light leakage in the xe2x80x9coffxe2x80x9d states. It would be beneficial to utilize commercially available and cost-effective waveplate retarders in a manner in which the Fresnel reflection could be readily eliminated without significant cost or complexity in manufacturing.
We have discovered that the conventional means for reducing the Fresnel reflection from a waveplate retarder is unsatisfactory in that an unmodulated reflected signal from a waveplate retarder associated with a particular color channel signal may ultimately degrade the contrast of a different color channel. This degradation was not only unexpected, but of a surprisingly significant magnitude for certain color channels in specific system configurations. Having developed a premise or theory for this contrast degradation, a further objective was then identified as the complete elimination of the Fresnel reflection. The objective of complete elimination of the Fresnel reflection in a waveplate retarder has heretofore been unappreciated among those skilled in art.
It is a further objective of the invention to completely eliminate the Fresnel reflection in a cost-effective manner.
It is a further objective of the invention to utilize readily available waveplate retarders of discrete thickness"".
It is a further objective of the invention to reduce the complexity and cost of assembling a projection display optical system with a waveplate retarder.
These and other objectives and features of the present invention will become more fully apparent from the following description and appended claims, or may be learned by the practice of the invention as set forth hereinafter.
Although one aspect of the invention is directed to waveplate retarders having values which are uniquely optimized by identifying and minimizing particular polarization attributes, other aspects are also beneficially directed towards conventional quarterwave plates.
The waveplate retarders are utilized in reflective imaging systems wherein light is xe2x80x9cdouble-passedxe2x80x9d through the optical paths of the system. The optical paths can be summarized as follows: light incident on a polarizing element is polarized to a first polarization state and then enters a color separation element typically a prism assembly. In the prism assembly, the polarized light undergoes color separation through the use of dichroic and reflective coatings on selected surfaces. The separate colors are emitted from the prism assembly to spatial light modulators, typically liquid crystal light valves, which change the polarization state of the reflected light in accordance with a desired image. The reflected light is passed, once again, through the prism assembly where the separate colors are recombined and the diverging light is emitted to a projection lens for display of the image on a screen.
The deleterious effects of the Fresnel reflection from the waveplate retarder on image contrast is eliminated by tilting the waveplate retarder so that its plane is not perpendicular to the optical axis it intersects. The reflected signal from the tilted retarder aligned in a first color channel is directed away from the entrance pupil of image forming optics (projection lens) corresponding to the other color channels so that the portion of the input signal from light source that is reflected by the waveplate retarder does not become a source of background noise decreasing the contrast and/or dark state color neutrality of the system. More specifically, when a waveplate retarder reflects green light from the light source, this reflected light is not redirected back to the entrance pupil of the image forming optics (projection lens) for either the red or blue color channel. While the waveplate retarder may only reflect from about 4% to about 0.1% percent of the input light this is a significant value when compared to the xe2x80x9coffxe2x80x9d state signal of the system. This is especially significant for the red and blue signal channels when the unmodulated signal is green light. The red or blue color channels will then suffer a substantial degradation in the color purity and contrast due to the much greater sensitivity of human vision to green light.